1. Field of the Invention
The present invention relates to methods of depositing piezoelectric films for example for use in methods of manufacturing Micro Electrical Mechanical Systems (MEMS), and in particular, RF MEMS devices such as thin film acoustic resonators used as filters in wireless and electrical circuits, where film thickness of film stack height affect the operational parameters of the system or device. Acoustic resonators contain a piezoelectric layer and at least a lower electrode, which together set a characteristic resonant frequency (which may be tuneable) enabling the resonators to be used as narrow band filters. More generally the invention relates to Micro Structure Technology (MST) devices which contain a mechanically resonant structure as may be found in ink jet heads, MEMS mirrors, accelerometers and gyroscopes and similar devices containing a vibrating component.
2. Background of the Invention
Bulk Acoustic Wave (BAW) resonators use longitudinal acoustic waves in thin piezoelectric material to filter signals and may be considered as the functional equivalent of a tank circuit. Fundamentally, there are two main designs in use for BAW resonators: Solidly Mounted Resonators (SMR's) and Film Bulk Acoustic Resonators (FBAR's). FIG. 1 shows a simplified graphic of the two approaches.
When an alternating electrical potential is applied, the piezoelectric layer will vibrate at a specific frequency converting some of the electrical energy into mechanical energy in the form of sound waves that propagate in the same direction as the electric field. At the mechanical resonance, the device also functions as an electrical resonator hence its ability to act as a filter. The mechanical resonant frequency is that for which the half wavelength of the propagated sound waves is a function of the total thickness of the resonant film.
It should be understood that the thickness accuracy and repeatability required for this application from wafer to wafer, deposition system to deposition system and over time greatly exceed those of the closely related semiconductor industry where 2%, 1 standard deviation wafer to wafer repeatability of thickness, is considered ‘state of the art’.
As the thickness of the piezoelectric material is determined at the point of manufacture then reworking is required to modify subsequently the film's thickness and hence the resonate frequency. In situ film thickness monitoring is impracticable in a close-coupled sputtering process preferred for depositing these materials.
Various methodologies have been proposed or are in use to ‘trim’ filters after production or modify their frequency of operation in service. U.S. Pat. No. 5,587,620 details some of these prior attempts and the use of an additional tuning layer 224 in an FBAR. This is an additional conductive layer on the underside of a silicon nitride layer 204 that increases the resonant thickness and thus lowers the resonant frequency of the FBAR type resonator. In post fabrication testing, the frequency is measured and material from layer 224 is evaporated by passing current through the layer 224 until the frequency increases to the desired value. An alternative method is offered there based on the same methodology of post manufacture testing and trimming of the resonant frequency by changing the thickness of the resonant structure of an FBAR. No solution is offered for an SMR, without etching the back of the wafer until it resembles an FBAR. These tuning processes are performed on an individual filter after it has been manufactured and built. They are therefore very expensive.
There is therefore a need for a methodology for trimming the resonant frequency of acoustic resonators during the manufacturing process whilst still at the wafer stage. It should be understood that each wafer may carry thousands or perhaps hundreds of thousands of filters, perhaps integrated within integrated circuits.